This invention relates to the field of two thread bag closing sewing machines and includes an improved looper assembly for two thread type machines and a process for two thread looping.
Two thread type sewing machines carry primary and secondary thread spools which supply the primary thread to the needle eye and secondary thread to a feed dog chamber located below the throat plate. This secondary thread is integrated with the primary thread during stitching to form the thread chain by interaction between a reciprocating needle and a looper assembly. In the known two thread bag closing sewing machines, the looper assembly utilizes a structurally complex thread carrying component which generally has a multiplicity of different and distinct mechanical movements. Typically the component is difficult and costly to form and follows a complex path including both swinging and thrusting movement in two or more intersecting planes on two or more sides of the needle. Because such structurally complex components and mechanical movements increase cost, require more operating space and increase the likelihood of failure and the frequencv of repair, it is highly desirable to provide a two thread looper assembly which is simple in configuration, easy to manufacture and utilizes simple, minimal mechanical movement within a single plane. None of the previously known two thread bag closing sewing machines utilizes a looper which moves along a simple arc within a single plane.
Still a further shortcoming of known two thread machines is that some utilize a looper which thrusts itself across the needle path at an angle of approximately ninety degrees to the direction of bag movement. This looper orientation can result in greater thread wear and breakage due to thread being first pulled normally in the direction of bag movement and then being sharply intercepted and pulled in a direction at right angles to the direction of bag flow.
While it is desirable to provide a simplified and improved two thread looper device which is usable with sewing machines of both the large stationary and the lightweight portable type, it is particularly crucial to have a streamlined, simple structure and movement for the looper of a two thread portable machine where weight and space are continually at a premium. The invention is particularly well adapted for use with portable bag closing machines of the general type shown in U.S. Pat. Nos. 3,094,955 and 4,348,970. These patents disclose a single thread portable bag closing sewing machine having a looper blade which moves in a single plane, but which is usable for single thread conventional stitching.
In recent years the need for two thread bag closing sewing machines has increased because of a growing need for double stitching of bags, and in some instances, specific government regulations requiring such stitching. The present invention provides an improved looper assembly which is simple in construction and moves along a simple arc wholly within a single plane on one side of the needle to eliminate the complex structure and movement associated with prior art two thread looping devices.